The use of conventional reverse osmosis (RO) technology for seawater desalination is neither new nor novel. Such technology has been demonstrated commercially both in the United States and elsewhere for over two decades. Several U.S. Government agencies were originally responsible for much of the early research funding in membrane desalination, with the result that many U.S. companies are now among the leaders in the production of membranes for desalination, both for seawater and brackish waters.
However, the high capital and operating costs of RO desalination of seawater have limited its application to situations where special circumstances justify its use. These circumstances generally amount to situations where there are limited alternatives for fresh water supply, such as shipboard desalting, or unusual economic situations such as in the Middle East where there is an abundance of low cost energy.
The reason for the wide-scale lack of acceptance of seawater desalting is basically economic; seawater desalting is expensive. The high cost for RO desalting is a function of its high capital cost and high operating cost. The capital costs are high for conventional seawater RO because of the high pressures which are used, typically 800 psi to 1200 psi, requiring high pressure pumping and piping systems which are costly. The high cost of exotic, corrosion-resistant materials, such as special alloy or stainless steels, which are generally used for most components, such as piping, valves, pumps, etc., and the need to use expensive membranes also increases the cost of seawater RO systems. Additionally, the design flux rates (e.g. gallons of permeate produced per square foot or square meter of effective membrane area) are typically quite low for commercial seawater RO membranes, requiring installations to have extended membrane-surface areas.
Seawater RO membranes are the "tightest" of the membranes used, which means that the rejection of salts is high but the flux of water is low in comparison to brackish water or nanofiltration membranes. When salt passage is low, as it is for such seawater RO membranes, single stage treatment will allow one to obtain potable quality water, i.e. less than about 500 ppm total dissolved solids (TDS), from seawater. However, the low specific water flux of such tight membranes, measured in GFD/psi (gallons of permeate per square foot of membrane area per day, per psi of net driving pressure) requires a large amount of effective membrane area and a high operating pressure to obtain adequate quantities of water.
The high operating pressure generally used in conventional seawater RO is thus a consequence of both (a) the inherently low specific flux of most seawater membranes and (b) the high osmotic pressure of seawater, which is typically about 360 psig for standard seawater having about 35,000 ppm. The combination of these design parameters has generally resulted in operating conventional seawater RO installations at a pressure in the range of about 800-1200 psi, with consequent high operating costs. Conversely, nanofiltration (NF) membranes operate at significantly lower pressures than RO membranes and have inherently high flux rates, typically 5-6 times higher than those for seawater RO membranes (0.11 GFD/psi vs. 0.02 GFD/psi).
The driving force for permeation for membrane separation is the net pressure across the membrane; this is defined as the feed pressure minus the permeate or back pressure, less the difference between the osmotic pressure of the feed and the osmotic pressure of the permeate. Because NF membranes allow high salt passage for monovalent ions, the osmotic pressure of the permeate is significant; this allows these membranes to partially desalt seawater while operating at pressure below the actual osmotic pressure of the feed.
U.S. Pat. No. 4,723,603 employs NF membranes for specific removal of sulfate from seawater. Sulfates are removed exceptionally well by NF membranes, and the NF permeate, still relatively rich in sodium chloride but deficient in sulfate, is used to formulate drilling mud on off-shore drilling rigs. Such sulfate-free water prevents the formation of barium sulfate which has extremely low solubility and can cause clogging.
U.S. Pat. No. 4,156,645 to Bray proposes to recover fresh water from seawater by a two-stage operation using a first stage with a "loose semipermeable membrane" to produce a water product containing 25 to 50% of the TDS of seawater, which permeate is treated in a subsequent stage using a "tight" semipermeable membrane at pressure between about 300 and 500 psi and results in a product having a TDS of less than 2,000 ppm.
My earlier U.S. Pat. No. 4,341,629 proposes to desalinate seawater by using two RO modules which can include the same membrane, e.g. a 90% rejection cellulose triacetate (CTA) RO membrane, or two different membranes, e.g. an 80% rejection CTA membrane and a 98% rejection CTA membrane.
U.S. Pat. No. 5,238,574 also shows the use of a multiplicity of RO membrane modules to separate seawater into potable water. For example, a first low-pressure RO membrane may be followed by a high pressure RO membrane, or a series of low pressure RO membranes can be used, to either provide permeate of varying water quality or simply to produce a combined permeate where the concentrate stream from one module becomes the feedstream for the next module in series. A generally similar arrangement is shown in U.S. Pat. No. 4,046,685 wherein the permeate streams are not combined; a higher quality water permeate stream is withdrawn separately from the first RO cartridge in series while the combined permeate streams from the next two RO cartridges in series are combined to produce a lower quality water product.
U.S. Pat. No. 5,458,781 discloses the production of a feedstream for the recovery of elemental bromine or alternatively for the production of metal bromide salt. Disclosed in Example 1 is the treatment of a brine source containing about 13,000 milligrams per liter of TDS which produces a desired RO concentrate or retentate, that is then treated in Example 2 with an NF membrane to produce an NF permeate which is high in bromide content and low in sulfate. In Example 3, the feedstream originally treated in Example 1 is instead treated using a nanofiltration module to very substantially increase the concentration of bromide and very substantially decrease the concentration of sulfate.
Although the methods and apparatus shown in the aforementioned U.S. patents may have had advantages in some particular instances, they have not proved to be a solution to the problem of how to efficiently and economically produce potable water from seawater. Thus, the search has continued for semipermeable membrane systems which can be used to produce potable water from seawater.